The present invention relates to an imaging device and, more particularly, to a method and device for driving a solid-state image sensor.
Recently, instead of conventional image pickup tube type image sensors, solid-state image sensors such as a charge-coupled device (CCD) and an MOS imaging element have been widely used in various technical applications such as an ITV camera and a video camera. In addition, a technique of laminating a photoconductive film on a photosensitive cell matrix section of a solid-state imaging element to obtain a highly sensitive image sensor has recently been noted by those skilled in the art. As compared with image pickup tube type image sensors, solid-state image sensors of these types are superior in size, weight, and operation reliability, and in terms of characteristics, they have an advantage of effectively suppressing image distortions, afterimages, and burnings. However, the solid-state image sensors have a problem of a "smear phenomenon". The smear phenomenon is a phenomenon in which excess carriers of signal charges, generated at a photosensitive cell matrix section in accordance with incidence of an optical image onto the image sensor, leak or mix with each other between adjacent cells of an array of photosensitive cells while they are transferred in a transfer section, thereby degrading the quality of a reproduced image.
In order to suppress or prevent the smear phenomenon, various sensor driving techniques have been proposed. For example, Japanese Patent Disclosure (Kokai) No. 58-17787 discloses a CCD image sensor, wherein excess carriers causing smear, generated in a vertical effective period of an image sensor for storing signal charges in accordance with incident image light, can be swept to an output section of a solid-state image sensor by applying a high-speed sweep pulse signal to a reset gate during a subsequent vertical blanking period, thereby suppressing the generation of smear. However, according to the image sensor operated in accordance with this driving method, in the vertical blanking period, an excessive output signal having a signal level several tens of times that of a normal image signal produced in the effective period is produced in the output section. Therefore, in order to accurately reproduce the excessive image output signal, a signal processor provided at an output stage of the image sensor must have a wide dynamic range. Furthermore, during a signal read operation in the next vertical effective period, a sag is generated in a sensor output signal due to the excessive sensor output signal, thereby posing another problem of degraded quality of a reproduced image.
Generally, in order to sweep the excess carriers to the output section of the image sensor in each vertical blanking period, a reset pulse having a constant phase (in-phase) is continuously applied to a reset gate throughout frame periods each consisting of the vertical blanking and vertical effective periods. In response to the application of the reset pulse signal, the excess carriers causing the smear are swept to a reset drain provided at the output stage of the image sensor in each vertical blanking period. Thus, the excessive sensor output signal including the excess carriers is produced in each vertical blanking period having no signal component. A signal processor for reproducing the excessive output signal must have a sufficiently wide dynamic range. A sag is generated in an image signal read out during the next vertical effective period. A sag in the output image signal generates vertical shading in a reproduced image, thereby degrading its quality.
According to another conventional driving method of reading a pixel signal, in order to sweep excess signal charges to the output section of the solid-state image sensor in each vertical blanking period, application of a reset pulse signal to the reset gate is completely stopped in only the vertical blanking period. According to this driving method, since the excess signal charges are directly swept to the reset drain of the image sensor in each vertical blanking period, no excessive sensor output signal is generated in an output terminal of the sensor. However, also in this case, a sag is generated in the image signal read out during the next vertical effective period. On the contrary, the generated sag has a level higher than that in the case of the above-mentioned driving method. This is because the reset pulse applied to the reset gate during the vertical blanking period is not at all mixed in the sensor output signal leaked through a stray capacitance (i.e., cut off), and a difference between a sensor output signal level obtained in the vertical blanking period and that including a mixed component of the reset pulse and obtained in the effective period is increased.